Conventionally, in the field of vacuum die casting, air is prevented from leaking into the cavity of the die out of the backside of the plunger tip. For example, JPA-2002-224807 discloses such an art.
However, in the conventional method disclosed in JPA-2002-224807, it may fail to close an opening of the end of a plunger sleeve and an opening of the molten metal inlet.
Concretely, with regard to the opening of the end of the plunger sleeve, the opening is closed by sliding a vacuum sleeve in the plunger sleeve, which results in deformation or expansion caused by the heat of the plunger sleeve. So, the distance, between the vacuum sleeve and plunger sleeve, changes; as a result, the seal performance will be degraded.
With regard to the opening of the molten metal inlet, the molten metal will be slopped around the inlet. So, the shutter or the like provided with the opening may fail to sufficiently seal the opening.
In the conventional method disclosed in JPA-2002-224807, in closing the openings of the end of the plunger sleeve and of the inlet, both of the space behind the plunger tip and the cavity are evacuated. However, there is a volume difference among the space and the cavity or there is a route resistance, so that it is difficult to keep constant the degrees of vacuum in the space and cavity; as a result, there exists a pressure differential therebetween. Unfortunately, this pressure differential will cause the penetration of the molten metal into the gap between the plunger tip and plunger sleeve or into the space behind the plunger tip. Accordingly, it causes galling at the plunger tip, sliding failure of the plunger tip or the like.
In the conventional vacuum die casting method, it is difficult to reach the required vacuum degree in the whole space including the cavity and plunger sleeve within the prescribed time due to the large volume of the cavity or the complex conduit to the cavity.
Considering the freezing of the molten metal in the plunger sleeve, the evacuation time may be around one second, so that there seems a variation in degrees of vacuum.
When the die casting is operated under the situation where the variation in the vacuum degree exists, the product will lack quality stability.